MORE THAN three years ago in this space, I described how The Chronicle attempted to reconcile one of the most hotly disputed aspects of political demonstrations -- crowd estimates.

At the time, anti-war forces were taking to the streets as the United States prepared its military drive into Iraq. The size of the gatherings was seen as a symbol of the power of the cause.

After a Jan. 18, 2003, protest left police and organizers miles apart in their judgments, The Chronicle sought an "objective" measure of the Feb. 16 demonstration by hiring an aerial photographer to produce detailed images of the route.

By placing a grid over each section, the paper could literally count heads.

It took days, but the results showed the numbers were far lower than both police and organizers had figured.

Fine, many readers said. So how are you going to handle future protests?

If the immigration rallies two weeks ago are any indicator, not well.

The numbers in the May 2 edition were a patchwork of guesstimates that made me wonder if the paper had any idea how big the crowds were.

But the big problem was the paper's front-page assertion that 30,000 people had marched in San Francisco. Against official estimates in the range of 400,000 in Chicago, 500,000 in Los Angeles and 75,000 in Denver, the San Francisco number prompted anger and bewilderment among protest backers.

You could discount the reaction as political sour grapes. A big crowd is a form of validation, so supporters want the number to be as high as possible, while a conservative estimate gives comfort to the other side.

The paper, on the other hand, must answer to different imperatives: consistency and credibility. By those measures, it didn't do itself proud.

There's no ideal way to count an ebbing, flowing crowd of people in an hours-long demonstration. Even a freeze-frame picture doesn't show how many people are passers-by or observers, and you don't know how many demonstrators are outside the field of vision. When is the peak moment? Do you count just the peak, or the total number of people over time?

But if there are no foolproof methods, there are plenty of bad ones. The paper managed to use several of them.

In the main news story, it said the Police Department and mayor's office would not provide a crowd estimate. The police have been resistant since the paper's aerial experiment, which was widely seen as an in your face to officers. So without an official count, the paper printed the 30,000 figure, provided by the Associated Press from an "unnamed city official."

By using the number, the paper violated its own policy on anonymous sources. The paper not only didn't know the source, it gave readers no clue as to why the source would know and why he or she didn't want to be named.

The figure, squishy as it was, made its way to the front page in a caption. But by then the qualifiers had been removed and the number was presented as fact.

Meanwhile, the paper decided not to use a crowd count from organizers, reasoning that the 500,000 figured offered up at one point was exaggerated. Perhaps that was a reasonable judgment, but it left the paper in the position of accepting the estimate of an unknown source over that of a known one.

In conversations with reporters, editors, organizers and readers, I've heard estimates ranging from the 30,000 figure to 80,000 to 100,000 to 200,000 and 500,000.

So what's a newspaper to do?

It can't very well start a story by saying, "People marched on Market Street yesterday." It can, however, give a sense of dimension. If demonstrators were shoulder-to-shoulder in Civic Center plaza and lined up along Market Street clear back to the Ferry Building, say that. If the crowd was sparse, outnumbered by lunchtime workers with brown bags, say that.

Then try to get the police on record. They're trained in dealing with crowds, and their estimates ought to be considered. If they won't give a figure, pester them to say why not. If they do hazard an estimate, get them to explain how they arrived at the number.

Organizers have the same responsibility. When they put out a number, they need to explain how they got it and let readers decide what to believe.

The approach doesn't get you precision, and it raises questions of its own, but it's a start. And it's necessary, because there's one thing you really can count on -- this won't be the last demonstration in San Francisco.